Estimation of Surface Energy of Non‐metallic Crystals from Hardness and Solubility Data

Abstract

At the present time there is evidence to show that hardness and surface free energy of crystals are related (DUNDON; ORMONT), and in fact they are closely related with the lattice energy (ORMONT ; PLENDL, GIELISSE) and energy of point-defect formation (SHUKLA, BAN-SIGIR). DUNDON fnund an apparent relationship between surface energy, y, and Mohs hardness, M , of several substances from the measured increase in the solubility of the salts. The relation between surface energy and supersaturation i.e. increase in solubility also follows from the Gibbs-Thomson equation (see e.g. MULLIN). However, since the surface energy of a crystal in vacuum is higher than that in contact with solution (SEARS; KLEBER), it is doubtful that the method of estimation of y from supersaturation gives precise values.Data on surface free energy are scanty (KUZNETSOV; GILMAN). In this note it is shown that surface energy of non-metallic crystals can conveniently be estimated from hardness and solubility data.Hardness is customarily expressed in Vickers, H,, or Knoop, H K , hardness number. The plot of Vickers hardness, H , in Vickers hardness number, and surface energy, y (dyne cm-I), of ionic crystals, illustrated in Figure 2, shows that the relationship between these quantities may be given by the empirical relation H , = 10-3yt.